High-frequency signal generator with low phase noise

ABSTRACT

A high-frequency oscillator comprises a reference-frequency generator and a high-frequency generator. The reference-frequency generator generates a variable reference frequency and supplies it to the high-frequency generator. The high-frequency generator comprises a phase-locked loop and generates a high-frequency signal from the variable reference frequency. The phase-locked loop comprises at least one first mixer, a second mixer and several switches. The first mixer, the second mixer and the switches are connected in series. The mixers are connected into the phase-locked loop individually in a selective manner by means of the switches.

CROSS-REFERENCE TO RELATED APPLICATION(S)

The present application is a national phase application of PCTapplication No. PCT/EP2011/072566, filed on Dec. 13, 2011, and claimspriority to German Patent Application No. DE 102011008350.2, filed onJan. 12, 2011, the entire contents of which are incorporated herein byreference.

FIELD

Embodiments of the present invention relate to high-frequency signalgenerators.

BACKGROUND

With conventional signal generators, a reference signal of variablefrequency is supplied to a phase-locked oscillator. This is thus excitedinto oscillation at an adjustable frequency.

Accordingly, DE 41 05 566 A1 discloses a mixer, which synchronizes anoscillator to a clear frequency in the GHz range. This mixer operateswith a multiplier diode, which generates a short pulse from thereference frequency, with the line spectrum of which the signal of theoscillator is mixed down. The disadvantage with this method is the poorsignal-to-noise ratio which can be achieved with this arrangement. Sincethe sampling mixer converts on all harmonics of the reference signal, alarge amount of noise is also mixed into the intermediate frequency.This restricts the sensitivity.

Moreover, US 2009/0309665 A1 discloses a high-frequency generator whichcontains a switchable phase-locked loop. Accordingly, the output signalof the oscillator is supplied optionally to a frequency splitter or aseries circuit of several mixers. The signal of the frequency splitteris used to adjust the oscillator to a coarse frequency. In order toimplement a fine adjustment, the arrangement then switches to the signalof the series-connected mixers. Even with a signal generator accordingto this design, it is only possible to achieve a sub-optimal phasenoise.

Accordingly, there is a need for a high-frequency signal generator,which achieves very good secondary-line spacing with low phase noise.

SUMMARY

Embodiments of the present invention, therefore, advantageously providefor a high-frequency signal generator that achieves improvedsecondary-line spacing with low phase noise.

According to an example embodiment of the present invention, a signalgenerator comprises two oscillators locked by means of phase-lockedloops. By way of example, a first phase-locked loop generates ahigh-quality reference frequency, which can be tuned in small, discretesteps over approximately 10% of the frequency. With this restrictedfrequency range, very good voltage-controlled oscillators can beconstructed for the purpose. With the use of a frequency splitteroutside the phase-locked loop, the phase noise of an originalfixed-frequency reference signal is largely preserved. The comparisonfrequency is advantageously around >10 MHz, so that rapid frequencychanges are possible. Passive doubling units with subsequent filteringare used in order to realize an extremely low-noise operation. As aresult of the advantageous filters between the doubling units,undesirable harmonics are suppressed.

To allow the mixing down of the voltage-controlled oscillator to theoutput frequency with different harmonics of the reference frequency,bridgeable mixers connected in a cascade are used in a further circuit.The output signal of the oscillators can be mixed down with thedifferent reference signals dependent upon the position of the bridgingswitch. This frequency range can be further increased by using themirror signal of the first mixer. The resulting intermediate frequencyis synchronized with a digital phase detector to a fraction of thereference frequency.

The use of the reference signal as an input signal for the splittermeans that crossing mixing products do not occur in the mixers. Themixing products are advantageously disposed on a matrix whichcorresponds to the last intermediate frequency divided by the resolutionof the splitter. Through an appropriate choice of the splitting factors,these mixing products can be selected in such a manner that the mixingproducts are suppressed by the loop filter and accordingly no secondarylines occur in the synthesizer.

To allow a very rapid frequency change, the oscillator is advantageouslypre-tuned. The tuning is based upon an individually measuredcharacteristic of the oscillator.

According to an example embodiment, a signal generator comprises areference-frequency generator and a high-frequency generator. By way ofexample, the reference-frequency generator generates a variablereference frequency and supplies it to the high-frequency generator. Thehigh-frequency generator comprises a phase-locked loop and generates ahigh-frequency signal from the variable reference frequency. Thephase-locked loop comprises at least one first mixer, a second mixer andseveral switches. The first mixer, the second mixer and the switches areconnected in series. The mixers are connected into the phase-locked loopindividually in a selective manner by the switches. Accordingly, anadjustability of the output frequency is achieved with a very low phasenoise.

By way of example, the phase noise can be further reduced by lowpassfilters after each mixer.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described by way of example in the following paragraphson the basis of drawings which illustrate advantageous exemplaryembodiments of the invention. The drawings show:

FIG. 1 illustrates a first partial view of a block-circuit diagram of anexemplary embodiment of the oscillator according to the invention; and

FIG. 2 illustrates a second partial view of a block-circuit diagram ofan exemplary embodiment of the oscillator according to the invention.

DETAILED DESCRIPTION

Initially, with reference to FIG. 1, the structure and method offunctioning of a reference-frequency generator is explained. Followingthis, the function of a high-frequency generator is described withreference to FIG. 2. The presentation and description of identicalelements in similar drawings have not been repeated in some cases.

In FIG. 1 and FIG. 2, an exemplary embodiment of the high-frequencyoscillator according to the invention is shown in two different views. Areference generator 1 contains a fractional frequency splitter 11, aphase detector 12, a loop filter 13, a voltage-controlled oscillator 14and a mixer 10. A stable-frequency reference signal of, for example, 640MHz, is supplied to the fractional frequency splitter 11. The fractionalfrequency splitter 11 generates a signal with a frequency divided byN_(Fref) and supplies it to the phase detector 12. The phase detector 12compares this signal with a signal generated by the mixer 10 and outputsa corresponding output signal to the loop filter 13. This filters thesignal and transmits it to the voltage-controlled oscillator 14. Thisgenerates an output signal of, for example, 650 MHz-700 MHz and suppliesit to the mixer 10 again. The latter mixes this output signal with astable-frequency reference signal.

In the case of a reference signal of, for example, 640 MHz, an outputsignal of the mixer of 10 MHz-60 MHz is obtained. The frequency of theoutput signal of the voltage-controlled oscillator 14 is accordinglyadjusted by setting the splitting factor N_(Fref) of the fractionalsplitter 11.

The reference-frequency generator 1 further comprises several frequencydoublers 15, 16, 17, 18, which double the frequency of a connectedsignal. A bandpass filter 20-23, which in each case allows only thedoubled frequency to pass and filters out the other components of thesignals, is connected downstream of each frequency doubler 15-18.Accordingly, a reference frequency signal of 650 MHz-700 MHz in theexample is present at the output of the voltage-controlled oscillator14. Accordingly, a doubled reference frequency of 1.3-1.4 GHz is presentat the output of the bandpass filter. A quadrupled reference frequencyof 2.6-2.8 GHz is present at the output of the bandpass filter 21. An8-fold reference frequency of 5.2-5.6 GHz is present at the output ofthe bandpass filter 22. A 16-fold reference frequency of 10.4-11.2 GHzis present at the output of the bandpass filter 23.

The doubled reference frequency of 1.3-1.4 GHz is supplied to afractional frequency splitter 30 of the high-frequency generator 2. Thisdivides the frequency of the signal by a splitting factor of N_(Fmain).As a result of the low intermediate frequency for the synchronization, ahigh quality, that is, a very low phase noise is achieved. Bymultiplying the frequency of the reference signal in small steps withsubsequent filtering, a very low phase noise of the reference frequencyis achieved.

The output signal of the fractional frequency splitter 30 is supplied toa phase discriminator 31, which compares it with the signal of aphase-locked loop 60 and further routes a corresponding output signal toa loop filter 32. The latter filters the signal and passes it to avoltage-controlled or current-controlled oscillator 33, advantageouslyan yttrium-iron-garnet (YIG) oscillator. The signal of the loop filter32 is used for the fine adjustment of the frequency of the controlledoscillator 33.

Furthermore, a signal is supplied from a coarse-control device 34 to thecontrolled oscillator 33 for a coarse adjustment of its outputfrequency. The output signal of the voltage-controlled oscillator 33 issupplied to the phase-locked loop 60 via a signal splitter 35. Itinitially passes through a mixer 36, by which it is mixed with the16-fold reference frequency. The output signal of the mixer is suppliedto a lowpass filter 41, which allows only the lower mixing product topass. The output signal is supplied to a switch 46, which optionallysupplies it to a further mixer 37 or bridges this mixer 37. If thesignal is supplied to the mixer 37, it is mixed with the 8-foldreference frequency of 5.2-5.6 GHz in the example. A further switch 47,which, together with the switch 46, implements the switching or thebridging of the mixer 37, is connected to the output of the mixer 37.

The resulting signal is supplied to a further bandpass filter 42 whichallows only the lower mixing product of the mixer 37 to pass. If themixer 37 has been bridged, the lowpass filter 42 plays no role for thesignal connected. The output signal is again supplied to a combinationof two switches 48, 49, which, like the switches 46, 47 either supplythe signal to a further mixer 38 or bridge the latter. If the signal issupplied to the further mixer 38, the latter mixes it with the 4-foldreference frequency of 2.6-2.8 GHz in the example. The output signal isagain supplied to a lowpass filter 43, which once again allows only thelower mixing product to pass. Here also, the filter 43 plays no role ifthe mixer 38 has been bridged.

Further switches 50, 51, a further mixer 39 and a further lowpass filter44 form another corresponding functional unit. The further mixer 39mixes with the doubled reference frequency of 1.3-1.4 GHz in theexample.

Further switches 52, 53, a further mixer 40 and a further lowpass filter45 form another corresponding functional unit. The further mixer 40mixes with an unchanged reference frequency of 650-700 MHz in theexample. The signal resulting after the lowpass filter 45 is supplied tothe phase discriminator 31.

The signals with which the mixers 36-40 mix the signal of thephase-locked loop 60 are taken from the reference-frequency generator 1.The first mixer 36 can also be advantageously provided with switches. Inthis case, this mixer can also be bridged. As an alternative, a largeror smaller number of mixers can also be used in the phase-locked loop.The higher the tuning range of the oscillator is supposed to be the moremixers are used.

The phase-locked loop 60 accordingly contains the mixers 36-40, thebandpass filters 41-45, the switches 46-53, the phase discriminator 31and the loop filter 32.

The following paragraphs explain how the splitting factors N_(Fref) andN_(Fmain) of the fractional splitters 11, 30 are adjusted in order toachieve a desired output frequency of the oscillator.

An output frequency of the oscillator of, for example, 10000 MHz to18000 MHz is taken as a starting point. Initially, the parameter V,which corresponds to the multiple of the reference frequency f_(ref)with which the mixing down is to be implemented, is calculated. Theminimal adjustable reference frequency of 650 MHz in the example and anintermediate frequency of, for example, 55 MHz, which is favorable forthe main loop, are used as a basis.V=INT((f _(osz)+55 MHz)/650 MHz)

Following this, the reference frequency f_(ref) is calculated. For thispurpose, the previously determined V is used. Since V is rounded down towhole numbers, a reference frequency somewhat higher than 650 MHz isobtained.f _(ref)=(f _(osz)+55 MHz)/V

The value of the splitter 11 of the reference-frequency generator is nowcalculated. The splitting factor N_(Fref) of the reference-frequencygenerator 1 is rounded in such a manner that no secondary lines occurwithin the loop bandwidth.N _(Fref)=640 MHz/ABS[(640−f _(ref))]with rounding to 1/F with

-   -   F=8 for N<20    -   F=4 for 20≦N<40    -   F=2 for 40≦N<80    -   F=1 for 80≦N

The rounding to different 1/F prevents the modulation from falling below8 MHz by a decimal component and accordingly being attenuated by thephase-locked loop.

Following this, the intermediate frequency f_(zf) in the phase-lockedloop 60 of the high-frequency generator 2 is calculated. Through therounding of the splitter 30 in generating the reference frequencyf_(ref), an intermediate frequency f_(zf) which differs from the setvalue is obtained.f _(zf) =V*640 MHz*(1−1/N _(Fref))−f _(OSZ)

This calculated intermediate frequency f_(zf) is now rounded to anadjustable value.N _(Fmain)=2*640 MHz*(1−1/N _(Fref))/f _(zf)with rounding of the splitting factor to 1/F with

-   -   F=16 for N<10    -   F=8 for 10≦N<20    -   F=4 for 20≦N<40    -   F=2 for 40≦N<80    -   F=1 for 80≦N

The rounding to different 1/F prevents the modulation from falling belowapproximately 8 MHz by the decimal component. The resulting secondarylines are accordingly suppressed by the phase-locked loop 60 of thehigh-frequency generator 1.

Finally, the actual frequency f_(OSZ) of the voltage-controlledoscillator 33 of the high-frequency generator 2 is calculated.f _(OSZ) =V*640 MHz*(1−1/N _(Fref))−(2*640 MHz*(1−1/N _(Fref))/N_(Fmain))

The residual error, which arises from rounding the splitting factorsN_(Fref), N_(Fmain), is smaller than 1 MHz and can be tolerated. Withthe advantageous use of a direct digital synthesizer in the phase-lockedloop 60 of the high-frequency oscillator 2 instead of the fractionalsplitter 30, an arbitrary frequency resolution would be possible.

The invention is not restricted to the exemplary embodiment presented.All of the features described or illustrated in the drawings can beadvantageously combined with one another as required within the scope ofthe invention.

The invention claimed is:
 1. A high-frequency oscillator comprising: areference-frequency generator; and a high-frequency generator, whereinthe reference-frequency generator is configured to generate a variablereference frequency and to supply the variable reference frequency atleast indirectly to the high-frequency generator, wherein thehigh-frequency generator comprises a phase-locked loop, wherein thehigh-frequency generator is configured to generate a high-frequencysignal at least indirectly from the variable reference frequency,wherein the phase-locked loop comprises a plurality of mixers, aplurality of switches, and a plurality of lowpass filters, wherein eachmixer is associated with one or more of the switches, and a one of thelowpass filters is located after each mixer, wherein each mixer and theassociated switches are configured in a manner whereby the mixer can beconnected into the phase-locked loop individually in a selective mannerby means of the switches.
 2. The high-frequency oscillator according toclaim 1, wherein the phase-locked loop comprises two switches per mixer,configured in a manner whereby, in a first switch position, the switchesconnect the respective mixer into the phase-locked loop, and, in asecond switch position, the switches bypass the respective mixer.
 3. Thehigh-frequency oscillator according to claim 1, wherein thereference-frequency generator is further configured to supply thevariable reference frequency or an integer multiple of the variablereference frequency to the mixers.
 4. The high-frequency oscillatorsaccording to claim 1, wherein the reference-frequency generator isfurther configured to supply the variable reference frequency to a firstof the plurality of mixers and a doubled variable reference frequency toa second of the plurality of mixers.
 5. The high-frequency oscillatoraccording to claim 1, wherein the phase-locked loop comprises N mixers,and wherein the reference-frequency generator is further configured tosupply an N²-fold variable reference frequency to the N mixers,respectively.
 6. The high-frequency oscillator according to claim 1,wherein the phase-locked loop comprises N mixers, and one frequencymultiplier for each of the N mixers, wherein each frequency multiplieris configured to convert the variable reference frequency to the to anN²-fold variable reference frequency and to supply the N²-fold variablereference frequency to an N-th one of the plurality of mixers.
 7. Thehigh-frequency oscillator according to claim 1, wherein the switches ofthe phase-locked loop are configured whereby at least one mixer isalways connected into the phase-locked loop.
 8. The high-frequencyoscillator according to claim 1, wherein the phase-locked loop furthercomprises a phase discriminator, wherein the phase discriminator isconfigured to compare a signal generated by the phase-locked loop with asignal derived from the variable reference frequency (f_(ref)).
 9. Thehigh-frequency oscillator according to claim 1, wherein: thehigh-frequency generator further comprises one of a voltage-controlledor a current-controlled oscillator, and a coarse control device, and thecoarse control device is configured to supply a coarse-control signal tothe controlled oscillator, and the phase-locked loop is configured tosupply a fine-control signal to the controlled oscillator.
 10. Thehigh-frequency oscillator according to claim 1, wherein thehigh-frequency generator further comprises a fractional frequencysplitter, wherein the fractional frequency splitter is configured todivide the frequency of a signal derived from the variable referencefrequency generated by the reference-frequency generator by a splittingfactor.
 11. The high-frequency oscillator according to claim 1, whereinthe high-frequency generator further comprises a power splitter and avoltage controlled oscillator, wherein the power splitter is configuredto supply a part of a high-frequency signal of the voltage-controlledoscillator to the phase-locked loop.
 12. The high-frequency oscillatoraccording to claim 1, wherein: the reference-frequency generatorcomprises an oscillator locked by means of a phase-locked loop and afrequency splitter, wherein the frequency splitter is configured todivide a reference signal of fixed frequency with a variable splittingfactor and supplies it and to supply the divided reference signal to thephase-locked loop, and the frequency of the high-frequency signal isconfigured to be adjusted by means of the variable splitting factor. 13.The high-frequency oscillator according to claim 12, wherein thereference-frequency generator further comprises at least two frequencydoublers and at least two bandpass filters, wherein: the lockedoscillator is connected to a first of the frequency doublers, a first ofthe bandpass filters is connected to the first frequency doubler, and toa second of the frequency doublers, the second frequency doubler isconnected to a second of the bandpass filters, the first bandpass filteris configured to output a doubled reference frequency (2*f_(ref)), andthe second bandpass filter is configured to output a quadrupledreference frequency (4*f_(ref)).
 14. The high-frequency oscillatoraccording to claim 1, wherein the reference-frequency generatorcomprises N series-connected frequency doublers and N associatedbandpass filters, wherein each of the N bandpass filters is configuredto output an N-fold reference frequency (N*f_(ref)).